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98 Volvo S90 heat blows cold in winter

My 98 S90 runs great (knock on wood) but in the winter the heater blows cold. Driving around town at lower speeds, it’s totally fine- oil temp gauge is right in the middle and heat flows. When I drive on the highway (say above ~50mph) if the temp is around 20F or below, the engine can’t maintain heat and therefore the heater blows cold. Is there an easy fix for this? Replacing thermostat? Temporary fixes? Thanks!

Sounds like thermostat

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But if there was something wrong with the thermostat, wouldn’t the car have trouble getting up to temp all the time?

are you reading the oil or water temperature gauge?

Sorry- just the temp gauge in the dash. I guess it’s coolant temp

No, in traffic or stopped it likely gets up to about the right temp. Drive faster and lots of air passes over the radiator and cools it below optimum.

Two things I can think of, assuming your thermostat works fine. 1. a problematic heater control valve and 2. possible a dirty heater core. A heater control valve can be easily replace. A dirty heater core can be back-flushed. Many times it works well and other times not so well.

If the coolant temp gauge isn’t reaching normal operating temperature when driving at freeway speeds, that’s usually a faulty thermostat. Thermostat’s have a tough life , what w/their living in coolant of varying temperatures, will almost always corrode over time, and it isn’t uncommon at all for them to fail in the open position. In extremely cold temperatures sometimes even a working thermostat can’t maintain the correct temperature, and vehicle owner have to resort to limiting the radiator’s air flow. But at 20 degrees F that shouldn’t be a problem. The only other thing I can think of is there’s air trapped in the cooling system. Do you hear any water-gurgling sounds?